The most significant limitation associated with the expanded use of nonmetallic adhesives for structural applications has been the absence of a reliable method of inspection. None of the conventional nondestructive inspection methods such as radiography, ultrasonics, and eddy current can be used to evaluate the general quality of a nonmetallic adhesive joint. Techniques developed for the evaluation of welded and brazed joints do not easily apply to "glued" construction considerations. For example, an epoxy bond between a rubber seal and a steel plate cannot be qualified by radiography because the epoxy cannot be seen; ultrasonics are unable to transmit high frequency sound waves across the interface; and eddy currents fail because of the non-conductivity of the adhesive bond and rubber. Very complicated thermal conductivity tests which may be used for inspection of an adhesive joint have proven unreliable. The importance of finding reliable ways in which to inspect the joints produced by such non-metallic adhesives was dramatically demonstrated by the loss of thermal heat shield tiles in several of the U.S. space shuttle missions.
One of the most promising developments in this field is smart structure technology. Smart structure technology refers to a material assessment concept including the implantation or attachment of sensors to a material to permit remote monitoring of material condition. Some options include the incorporation of optical or acoustic wave guides (glass or wire strands) in reinforced plastic composite components. Periodic or continuous monitoring of the wave guide response is correlated with material integrity and consequently provides a unique nondestructive test capability. While demonstrated successfully for a number of aerospace structure applications, the wave guide approach is limited by two major problems: (1) only materials near the vicinity of the wave guide can be evaluated and (2) prior knowledge of where to place the wave guide is critical to system success.
In 1984 Westinghouse scientists developed a version of the smart structure concept for the inspection of adhesive joints. In this version of the smart structure concept, a nondestructive method of forming and inspecting an adhesive joint to ensure the presence of adhesive and provide a reliable joint between two joined pieces is disclosed. The method includes tagging a nonmetallic adhesive material to be used for a joint by evenly dispersing a small amount of finely-divided ferromagnetic particles throughout the adhesive material. The tagged adhesive material is applied to the two pieces to form the adhesive joint. A magnetic permeability detector device such as an eddy current probe is passed over the surface of one of the pieces in the area of the joint to scan the joint to detect changes in magnetic permeability in the joint area. The detected magnetic permeability of the scanned joint area is recorded to form a magnetic permeability map of the scanned joint area. The map of the scanned joint area is visually examined for relatively low magnetic permeability locations compared to the remaining portions of the scanned joint area. Low magnetic permeability locations indicate voids in the joint. An alternative method includes soaking the adhesive joint with a ferrofluid dye penetrant and passing a magnetic permeability detector over the surface of one of the pieces in the area of the joint to scan the joint to detect changes in magnetic permeability in the joint area. The detected magnetic permeability of the scanned joint area is recorded to form a magnetic permeability record of the scanned joint area. Relatively high magnetic permeability locations indicate voids in the joint where the ferrofluid dye has penetrated.
FIGS. 1A and 1B illustrate the basic principle involved in this method. This simple test is used to determine the presence of the adhesive - an absolutely critical bond integrity parameter. Variations in this concept have been proposed for the assessment of a wide variety of materials where the absence or presence of the tagged particles can be used to indicate material integrity. The technique has been explored for the assessment of composite, caulking, and insulation systems as well as powder processing and foam core structures. Although the ferromagnetic tagging concept can provide important information regarding material continuity, the applicants have observed that tagging alone cannot reflect bond strength.
A recent development involving tagging combined with the utilization of an electromagnetic field probe and vibration monitor has led the inventors to a unique nondestructive approach to the qualitative (and perhaps quantitative) assessment of bond integrity in adhesive joints and overall structural integrity in composite materials as explained below.